1. Field of the Invention
The present invention relates to a CRT display unit. More particularly, the present invention relates to a CRT display unit which can be used in two modes, that is, a standard mode having 262.5 scanning lines for one field and a horizontal deflecting frequency of 15.754 kHz and a high-resolution mode having scanning lines twice as many as those of the standard mode and a horizontal deflecting frequency of approximately 25 kHz, this CRT display unit being utilizable as a display unit for a personal computer, a home video game set and the like.
2. Description of the Prior Art
Recently, in order to display a clear picture in an electronic calculator such as a personal computer, improvements have been made to enhance the performance of the graphic function. In fact, there are personal computers which provide an output having a resolution of 320.times.200 dots, 640.times.200 dots or 640.times.400 dots. As a CRT display unit corresponding to such graphic display, a television set for domestic use can be used in the case of 320.times.200 dots or 640.times.200 dots since the scanning lines in such cases are less than 230 scanning lines generally included in an ordinary television set put on the market.
On the other hand, in a CRT display unit for display at a high resolution with 640.times.400 dots, the number of scanning lines is increased and consequently, it is necessary in principle to lower the vertical deflecting frequency in this case. However, practically, the vertical deflecting frequency cannot be much lowered because of the necessity of preventing flicker in the Braun tube. For example, in the case of 200 lines and 60 Hz, the vertical deflecting frequency can only be decreased to approximately 50 to 55 Hz. In consequence, it is necessary to increase the horizontal deflecting frequency, for example, to 24.85 kHz with respect to 15.75 kHz with 200 lines. Such vertical and horizontal deflecting frequencies are greatly different from those of an ordinary television set and therefore, a high-resolution CRT display unit to be used exclusively for display is required. Display units of 400 lines meeting such requirement, especially those for personal use are in little demand and the manufacturing cost thereof tends to be high.